Effect of intrinsic-gain fluctuations on quantum noise of phosphor materials used in medical X-ray imaging

Abstract

The quality of a medical image depends, among other parameters, on quantum noise. Quantum noise is affected by the fluctuations in the number of optical quanta produced within the phosphor, per absorbed X-ray (i.e. phosphor intrinsic-gain fluctuations). This effect is considered by means of a factor, called in this study intrinsic-gain noise factor, IGNF(E). In existing theoretical models of quantum noise, the corresponding factor is taken to be equal to one. In this paper, an expression that accounts for the coefficient of variation of the phosphor intrinsic gain is introduced. This expression takes into account the process of electron-hole pair conversion to optical photons and the frequency distribution function of the emitted optical photon energy. Subsequently IGNF(E) is expressed in terms of this coefficient of variation. IGNF(E) has been calculated for several phosphors and for various energies. For all medical X-ray energies studied, phosphors that exhibit a high relative fluctuation of emitted optical photon energy, IGNF(E) exceeds by 2% to over 17% the corresponding factor of the existing theoretical models of quantum noise. PACS: 78.65; 42.80Phosphors doped with various activators (Eu, Tb, Ag, etc.) are widely used in medical imaging as X-ray-to-light converters in conjunction with optical detectors (films, photodiodes, etc.) [1][2][3]. The quality of a medical image depends, among other parameters, on quantum noise. Quantum noise is affected by the number of optical quanta, produced within the phosphor, per absorbed X-ray of energy E (i.e. phosphor intrinsic gain) [4,5]. Phosphor intrinsic gain is subject to statistical fluctuations. Thus for equal X-ray energy absorption, unequal number of optical quanta are produced [4][5][6][7]. Quantum noise is proportional to the variation of the intrinsic phosphor gain, which reflects the statistical fluctuations in the production of optical quanta produced by the activator material within the phosphor. Experimental methods have reported to determine the fluctuation of the optical quanta emitted by the phosphor per absorbed X-ray [8-10]. However, to the best of our knowledge, no experimental method has been introduced, to measure the fluctuation of intrinsic phosphor gain. In addition, the contribution to quantum noise, called in this study intrinsic-gain noise factor (IGNF(E)), in existing theoretical models, of the fluctuation of phosphor intrinsic gain, is unity, as the gain procedure is assumed to follow Poisson distribution [2,[4][5][6]11].In this paper, an alternative expression of phosphor intrinsic-gain fluctuation is presented, which can contribute to quantum-noise modeling. This expression takes into account the process of electron-hole pair conversion to optical photons and the frequency distribution function of the emitted optical photon energy. IGNF(E) incorporating the above expression has been calculated for several phosphor materials and for various energies, based on X-ray conversion efficiency and optical emission spectral data. Method and mate...

A comparative investigation of Lu<inf>2</inf>SiO<inf>5</inf>:Ce and Gd<inf>2</inf>O<inf>2</inf>S:Eu phosphor scintillators for use in a medical imaging detectors

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Evaluating optical spectral matching of phosphor-photodetector combinations

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“…Where var [E] λ is the statistical variance of the emitted light photon spectrum [3], [4], hence OGTE is, practically, affected only by the inherent and effective optical photon energy distributions.…”

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The effect of terbium concentration on the luminescent properties of yttrium oxysulfide phosphor for FED application

“…In 1977 and 1978, Gutzler investigated the efficiency of the phosphor under low-energy electron bombardment [4,5]. Y 2 O 2 S:Tb thin film and bulk material were synthesized and the luminescent properties and temperature dependence of cathodoluminescence were investigated [6][7][8] applications of X-ray imaging [10][11][12]. However, details of relationship between the doping concentration of terbium in yttrium oxysulfide have not been investigated.…”